A Method for Simulating Pressure/Production Performance of Volumetric Dry Gas Reservoirs

Abstract

Summary This paper presents a method to simulate the pressure/production behavior of closed, dry-gas reservoirs. The simulation model contains one, two, or three producing regions, each of which may be subdivided into as many as 10 noncommunicating layers. Flow between the layers occurs only through the wellbore. In each region, permeability, porosity, thickness, and turbulence coefficients are entered for each layer; each layer is assumed to exist in all regions. With free flow, at any instant, the flow capacity of each region is the rate at which the sum of the pressure drops in the formation, the production tubing, and the surface flowline equals the difference between the reservoir and surface discharge pressures. With compression, the sum of the flow capacities from the regions must correspond to the compression ratio attainable with the horsepower in place. Introduction First, we consider the physical effects of the simple reservoir model (SRM). Next, the model's geometric configuration and important computational features are described, and the calculational sequence is explained with the help of a flow diagram. Types of input data are listed, and a solution to an example problem is given, along with a discussion of our computing experience. Physical Effects Fig. 1 shows the reservoir subdivided into three regions. Each region's vertical cross section may be broken into 10 layers, with each layer in each region having its own porosity, permeability, and thickness. Each "average" well in a region penetrates and produces from every layer. In Fig. 2, produced gas enters the wellbore, flows up the tubing and through the gathering flowline and the separator/dehydrator; then, the gas flows either to the compressor or directly into the trunkline. The SRM tracks each layer's and region's reservoir pressure and production capacity as a function of cumulative production. The three-cell model appropriately balances computing time and realism of predictions. In the dry-gas reservoirs considered, formation transmissibility is sufficiently large and viscosity sufficiently small so that little accuracy is sacrificed by not using more cells. The physical effects considered by the SRM are as follows.Pressure vs. cumulative production in each layer in each region.In each layer, gas flow rate between regions vs. pressure difference.Gas flow rate into the wellbore from each layer vs. layer pressure, bottomhole flowing pressure (BHFP), and number of wells. The total regional production rate into the separator equals the sum of the layer rates.Pressure drop in the tubing vs. the region's production rate.Pressure drop in surface flowlines vs. the region's production rate.Pressure drop across the field separator.Field separator overhead and bottom flow rate vs. inlet rate. The overhead is the trunkline inlet gas.Fraction of produced gas burned as field fuel (a function of the total capacity of all field equipment).Trunkline inlet rate vs. compressor suction pressure, trunkline inlet pressure (input), and installed horsepower in each stage. Important Computational Features The SRM comprises five groups of equations, whose mathematical forms are given in the Appendix, along with a count of the number used in each particular model and an examination of solution methods. Material Balance. The ni´nr material balance equations account for the production from each layer in each region and for the flow between regions in each layer.